In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the unimaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is positive working. Conversely, if the unimaged regions are removed, the precursor is negative working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaged imageable elements typically require processing in a developer to convert them to lithographic printing plates. Developers are typically aqueous alkaline solutions, which may also contain substantial amounts of organic solvents. Because of their high pH and the presence of organic solvents, disposal of substantial quantities of used developer is expensive and can cause environmental problems.
On-press developable lithographic printing plate precursors can be directly mounted on a press after imaging and developed with ink and/or fountain solution during the initial press operation. These precursors do not require a separate development step before mounting on press. On-press imaging, in which the precursor is both imaged and developed on press, eliminates mounting the precursor in a separate imaging device.
The substrate typically comprises a support of aluminum or an aluminum alloy that has been treated to form a layer of aluminum oxide of its surface. The substrate may also comprise a layer on one or both surfaces to modify the surface characteristics to enhance the hydrophilicity of the surface of the support, to improve adhesion to subsequent layers to the substrate, and to protect the oxide layer during the development process. Although silicates have a good ink scumming properties, that is, they typically do not cause scumming of the printed image, they typically have poor adhesion to the overlying layer, typically the imageable layer. To overcome this deficiency, a diazo polymer is sometimes added to the imageable layer. However, the diazo polymer increases on-press development time and also reduces the shelf life of the lithographic printing plate precursor.
Consequently, a polyvinylphosphonic acid (PVPA) layer is typically used instead of a silicate layer. However, substrates that comprise polyvinylphosphonic acid layers are more prone to scumming than those that comprise silicate layers. Thus, a need exists for substrates for lithographic printing plate precursors, especially for on-press developable lithographic printing plate precursors, that have good adhesion to the overlying layer, do not increase on-press development time, and do not cause scumming of the printed image.